linux-kernel - Re: [PATCH 7/7 v3] sched: fix wrong utilization accounting when switching to fair class

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Date:   Fri, 16 Sep 2016 12:51:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        yuyang.du@...el.com, Morten.Rasmussen@....com,
        linaro-kernel@...ts.linaro.org, dietmar.eggemann@....com,
        pjt@...gle.com, bsegall@...gle.com
Subject: Re: [PATCH 7/7 v3] sched: fix wrong utilization accounting when
 switching to fair class

On Mon, Sep 12, 2016 at 09:47:52AM +0200, Vincent Guittot wrote:

> -dequeue task
> -put task
> -change the property
> -enqueue task
> -set task as current task

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3e52d08..7a9c9b9 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1105,10 +1105,10 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
>  
>  	p->sched_class->set_cpus_allowed(p, new_mask);
>  
> -	if (running)
> -		p->sched_class->set_curr_task(rq);
>  	if (queued)
>  		enqueue_task(rq, p, ENQUEUE_RESTORE);
> +	if (running)
> +		p->sched_class->set_curr_task(rq);
>  }
>  
>  /*

So one thing that I've wanted to do for a while, but never managed to
come up with a sensible way to do is encapsulate this pattern.

The two options I came up with are:

#define FOO(p, stmt)
({
	struct rq *rq = task_rq(p);
	bool queued = task_on_rq_queued(p);
	bool running = task_current(rq);
	int queue_flags = DEQUEUE_SAVE; /* also ENQUEUE_RESTORE */

	if (queued)
		dequeue_task(rq, p, queue_flags);
	if (running)
		put_prev_task(rq, p);

	stmt;

	if (queued)
		enqueue_task(rq, p, queue_flags);
	if (running)
		set_curr_task(rq, p);
})

and

void foo(struct task_struct *p, void (*func)(struct task_struct *, int *))
{
	struct rq *rq = task_rq(p);
	bool queued = task_on_rq_queued(p);
	bool running = task_current(rq);
	int queue_flags = DEQUEUE_SAVE; /* also ENQUEUE_RESTORE */

	if (queued)
		dequeue_task(rq, p, queue_flags);
	if (running)
		put_prev_task(rq, p);

	func(p, &queue_flags);

	if (queued)
		enqueue_task(rq, p, queue_flags);
	if (running)
		set_curr_task(rq, p);
}

Neither results in particularly pretty code. Although I suppose if I'd
have to pick one I'd go for the macro variant.

Opinions? I'm fine with leaving the code as is, just wanted to throw
this out there.